Bow-shock Pulsar Wind Nebula Research Articles

The TRAPUM Small Magellanic Cloud pulsar survey with MeerKAT – I. Discovery of seven new pulsars and two Pulsar Wind Nebula associations

ABSTRACT The sensitivity of the MeerKAT radio interferometer is an opportunity to probe deeper into the population of rare and faint extragalactic pulsars. The TRAPUM (TRAnsients and PUlsars with MeerKAT) collaboration has conducted a radio-domain search for accelerated pulsars and transients in the Small Magellanic Cloud (SMC). This partially targeted survey, performed at L band (856–1712 MHz) with the core array of the MeerKAT telescope in 2-h integrations, is twice as sensitive as the latest SMC radio pulsar survey. We report the discovery of seven new SMC pulsars, doubling this galaxy’s radio pulsar population and increasing the total extragalactic population by nearly a quarter. We also carried out a search for accelerated millisecond pulsars in the SMC globular cluster NGC 121 using the full array of MeerKAT. This improved the previous upper limit on pulsed radio emission from this cluster by a factor of 6. Our discoveries reveal the first radio pulsar-PWN systems in the SMC, with only one such system previously known outside our Galaxy (the ‘Crab pulsar twin’ in the Large Magellanic Cloud, PSR J0540−6919). We associate the 59 ms pulsar discovery PSR J0040−7337, now the fastest spinning radio pulsar in the SMC, with the bow-shock Pulsar Wind Nebula (PWN) of supernova remnant DEM S5. We also present a new young pulsar with a 79 ms period, PSR J0048−7317, in a PWN recently discovered in a MeerKAT radio continuum image. Using the multibeam capability of MeerKAT, we localized our pulsar discoveries, and two previous Murriyang discoveries, to a positional uncertainty of a few arcseconds.

The nature of the X-ray filaments around bow shock pulsar wind nebulae

Context. We propose that the X-ray filaments emerging from selected bow shock pulsar wind nebulae are due to a charge-separated outflow of electrons and/or positrons escaping the nebula and propagating along the local Galactic magnetic field. Aims. The X-ray brightness, length, and thickness of filaments are all accounted for if a nonresonant streaming instability is excited. Methods. This is possible if particles are released in the interstellar medium as a collimated beam, as would be expected in a reconnection region between the nebular and interstellar magnetic fields. Results. We successfully test this idea on the Guitar Nebula filament and discuss other cases. Conclusions. These filaments provide the best diagnostics available for particle escape from evolved pulsar wind nebulae, a process essential to assessing the contribution of these sources to cosmic ray positrons. The same phenomenology might govern the occurrence of TeV halos and their importance for cosmic ray transport.

Fast as Potoroo: Radio continuum detection of a bow-shock pulsar wind nebula powered by pulsar J1638–4713

Abstract We report the discovery of a bow-shock pulsar wind nebula (PWN), named Potoroo, and the detection of a young pulsar J1638 $-$ 4713 that powers the nebula. We present a radio continuum study of the PWN based on 20-cm observations obtained from the Australian Square Kilometre Array Pathfinder (ASKAP) and MeerKAT. PSR J1638 $-$ 4713 was identified using Parkes radio telescope observations at frequencies above 3 GHz. The pulsar has the second-highest dispersion measure of all known radio pulsars (1 553 pc cm $^{-3}$ ), a spin period of 65.74 ms and a spin-down luminosity of $\dot{E}=6.1\times10^{36}$ erg s $^{-1}$ . The PWN has a cometary morphology and one of the greatest projected lengths among all the observed pulsar radio tails, measuring over 21 pc for an assumed distance of 10 kpc. The remarkably long tail and atypically steep radio spectral index are attributed to the interplay of a supernova reverse shock and the PWN. The originating supernova remnant is not known so far. We estimated the pulsar kick velocity to be in the range of 1 000–2 000 km s $^{-1}$ for ages between 23 and 10 kyr. The X-ray counterpart found in Chandra data, CXOU J163802.6 $-$ 471358, shows the same tail morphology as the radio source but is shorter by a factor of 10. The peak of the X-ray emission is offset from the peak of the radio total intensity (Stokes $\rm I$ ) emission by approximately 4.7 $^{\prime\prime}$ , but coincides well with circularly polarised (Stokes $\rm V$ ) emission. No infrared counterpart was found.

Chandra X-Ray Observatory Observations of 13 Fermi LAT Sources

In the latest data release from the Fermi Gamma-ray Space Telescope (the 4th Fermi LAT 14 yr Catalog, or 4FGL), more than 50% of the Galactic sources are yet to be identified. We observed 13 unidentified Fermi LAT sources with the Chandra X-Ray Observatory to explore their nature. We report the results of the classification of X-ray sources in the fields of these γ-ray sources and discuss the implications for their nature. We use multiwavelength (MW) data for a machine-learning classification, accompanied by a more detailed spectral/variability analysis for brighter sources. Eight 4FGL sources have γ-ray pulsars within their position error ellipses. We consider three of these pulsars (PSR J1906+0722, PSR J1105–6037, and PSR J1358–6025) to be detected in X-rays, while PSR J1203–6242 shows a hint of X-ray emission. Within the positional uncertainties of three of the 4FGL sources, we detect X-ray sources that may be yet unknown pulsars, depending on the MW association. In addition to point sources, we discovered two extended sources, one of which is likely to be a bow-shock pulsar-wind nebula associated with PSR J1358–6025. Finally, we classify other X-ray sources detected in these observations and report the most interesting classifications.

Resolving the Bow Shock and Tail of the Cannonball Pulsar PSR J0002+6216

We present X-ray and radio observations of the recently discovered bow-shock pulsar wind nebula (PWN) associated with PSR J0002+6216, characterizing the PWN morphology, which was unresolved in previous studies. The multifrequency, multiepoch Very Large Array (VLA) radio observations reveal a cometary tail trailing the pulsar and extending up to 5.′3, with multiple kinks along the emission. The presented radio continuum images from multiconfiguration broadband VLA observations are one of the first results from the application of multiterm multifrequency synthesis deconvolution in combination with the AWProject gridder implemented in the Common Astronomy Software Applications (CASA) package. The X-ray emission observed with Chandra extends to only 21″, fades quickly, and has some hot spots present along the extended radio emission. These kinks could indicate the presence of density variations in the local interstellar medium or turbulence. The bow-shock standoff distance estimates a small bow-shock region with a size of 0.003–0.009 pc, consistent with the pulsar spin-down power of = 1.51 × 1035 erg s−1 estimated from timing. The high-resolution radio image reveals the presence of an asymmetry in the bow-shock region, which is also present in the X-ray image. The broadband radio image shows an unusually steep spectrum along with a flat-spectrum sheath, which could indicate varying opacity or energy injection into the region. Spatially resolved X-ray spectra provide marginal evidence of synchrotron cooling along the extended tail. Our analysis of the X-ray data also shows that this pulsar has a low spin-down power and one of the lowest X-ray efficiencies observed in these objects.

3D Relativistic MHD Simulations of Pulsar Bow Shock Nebulae

Pulsars out of their parent SNR directly interact with the ISM producing so called Bow-Shock Pulsar Wind Nebulae, the relativistic equivalents of the heliosphere/heliotail system. These have been directly observed from Radio to X-ray, and are found also associated to TeV halos, with a large variety of morphologies. They offer a unique environment where the pulsar wind can be studied by modelling its interaction with the surrounding ambient medium, in a fashion that is different/complementary from the canonical Plerions. These systems have also been suggested as the possible origin of the positron excess detected by AMS and PAMELA, in contrast to dark matter. I will present results from 3D Relativistic MHD simulations of such nebulae. On top of these simulations we computed the expected emission signatures, the properties of high energy particle escape, the role of current sheets in channeling cosmic rays, the level of turbulence and magnetic amplification, and how they depend on the wind structure and magnetisation.

A deep X-ray spectral imaging of the bow-shock pulsar wind nebula associated with PSR B1929+10

In this work, we study the X-ray bow-shock nebula powered by the mature pulsar PSR B1929+10 using data from XMM-Newton, with an effective exposure of ∼300 ks, offering the deepest investigation of this system thus far. We found the X-ray axial outflow extends as long as ∼8 arc minute behind the proper motion direction, which is a factor of two longer than the result reported in the previous study. Furthermore, we found evidence of two faint lateral outflows extending laterally with respect to the proper motion. We also found indications of spectral hardening along the axial outflow, suggesting that certain acceleration processes might occur along this feature.

On the origin of jet-like features in bow shock pulsar wind nebulae

ABSTRACT Bow shock pulsar wind nebulae are a large class of non-thermal synchrotron sources associated to old pulsars that have emerged from their parent supernova remnant and are directly interacting with the interstellar medium. Within this class a few objects show extended X-ray features, generally referred as ‘jets’, that defies all the expectations from the canonical MHD models, being strongly misaligned respect to the pulsar direction of motion. It has been suggested that these jets might originate from high energy particles that escape from the system. Here we investigate this possibility, computing particle trajectories on top of a 3D relativistic MHD model of the flow and magnetic field structure, and we show not only that beamed escape is possible, but that it can easily be asymmetric and charge separated, which as we will discuss are important aspects to explain known objects.

Full-3D relativistic MHD simulations of bow shock pulsar wind nebulae: emission and polarization

ABSTRACT Bow shock pulsar wind nebulae are observed with a variety of complex morphologies at different wavelengths, most likely due to differences in the magnetic field strength and pulsar wind geometry. Here we present a detailed analysis, showing how these differences affect the observational properties in these systems, focusing on non-thermal synchrotron emission. By adopting different prescriptions for the local emissivity, on top of the magnetic and flow patterns taken from three-dimensional (3D) high-resolution numerical simulations in relativistic magnetohydrodynamics (MHD), and considering various viewing angles, we try to characterize the main features of the emission and polarization, to verify if and how these can be used to get information, or to put constraints, on known objects. We found, for example, that conditions leading to a strong development of the turbulence in the bow shock tail produce substantial differences in the emission pattern, especially in polarized light.

Probing the origin of the off-pulse emission from the pulsars B0525+21 and B2045−16

Pulsars typically exhibit radio emission in the form of narrow pulses originated from confined regions of their magnetospheres. A potential presence of magnetospherically originated emission outside this region, the so-called off-pulse emission, would challenge the existing theories. Detection of significant off-pulse emission has been reported so far from only two pulsars: B0525+21 and B2045−16 at 325 and 610 MHz, respectively. However, the nature of this newly uncovered off-pulse emission remains unclear. To probe its origin we conducted very-high-resolution radio observations of B0525+21 and B2045−16 with the European VLBI Network (EVN) at 1.39 GHz. Whereas the pulsed emission is detected at a level consistent with previous observations, we report an absence of any off-pulse emission above 42 and 96 μJy beam−1 (three times the rms noise levels) for B0525+21 and B2045−16, respectively. Our stringent upper limits imply the off-pulse emission to be less than 0.4 and 0.3% of the period-averaged pulsed flux density, i.e., much fainter than the previously suggested values of 1−10%. Since the EVN data are most sensitive to extremely compact angular scales, our results suggest a non-magnetospheric origin for the previously reported off-pulse emission. The presence of extended emission that is resolved out to these milliarcsecond scales still remains plausible. In this case, we constrain the emission to arise from structures with sizes of ∼(0.61−19) × 103 au for B0525+21 and ∼(0.48−8.3) × 103 au for B2045−16. These constraints might indicate that the two pulsars are accompanied by compact bow-shock pulsar wind nebulae. Future observations probing intermediate angular scales (∼0.1−5 arcsec) will help to clarify the actual origin of the off-pulse emission.

3D relativistic MHD simulations of bow-shock Pulsar Wind Nebulae with highly asymmetric geometry

Bow-shock pulsar wind nebulae (PWNe) show a variety of morphological shapes. We attribute this diversity to the geometrical factors: relative orientations of the pulsar rotation axis, proper velocity, and the line of sight. Here we report study of morphology in the most complicated geometry, when the pulsar rotation axis and its proper velocity makes an arbitrary angle (in the simulations we adopt a value of ∼ π/4). Using 3D RMHD simulations we obtain the MHD structure of PWN and compute synthetic synchrotron emissivity maps.

The Tail of PSR J0002+6216 and the Supernova Remnant CTB 1

Abstract We have carried out Very Large Array imaging and a Fermi timing analysis of the 115 ms γ-ray and radio pulsar PSR J0002+6216. We found that the pulsar lies at the apex of a narrowly collimated cometary-like 7′ tail of nonthermal radio emission, which we identify as a bow-shock pulsar wind nebula. The tail of the nebula points back toward the geometric center of the supernova remnant CTB 1 (G116.9+0.2) 28′ away, at a position angle θ μ = 113°. We measure a proper motion with 2.9σ significance from a Fermi timing analysis giving μ = 115 ± 33 mas yr−1 and θ μ = 121° ± 13°, corresponding to a large transverse pulsar velocity of 1100 km s−1 at a distance of 2 kpc. This proper motion is of the right magnitude and direction to support the claim that PSR J0002+6216 was born from the same supernova that produced CTB 1. We explore the implications for pulsar birth periods, asymmetric supernova explosions, and mechanisms for pulsar natal kick velocities.

Full-3D relativistic MHD simulations of bow shock pulsar wind nebulae: dynamics

Bow shock pulsar wind nebulae (BSPWNe) are know to show a large variety of shapes and morphologies, both when comparing different objects, and for the same object in different energy bands. It is unclear if such a variety is related to differences in the pulsar wind properties, or to differences in the conditions of the ambient medium. We present here a set of full three-dimensional, relativistic and magneto-hydrodynamic simulations of BSPWNe, with the intention of determining how differences in the injection conditions by the pulsar wind reflect in the nebular dynamics. To achieve a good coverage of the available parameter space we have run several simulations varying those parameters that are most representative of the wind properties: the latitudinal anisotropy of the wind energy flux with respect to the pulsar spin axis, the level of magnetization, the inclination of the pulsar spin axis with respect to the pulsar direction of motion. We have followed the dynamics in these systems, not just in the very head, but also in the tail, trying to assess if and how the system retains memory of the injection at large distances from the pulsar itself. In this paper we focus our attention on the characterization of the fluid structure and magnetic field properties. We have tried to evaluate the level of turbulence in the tail, and its relation to injection, the survival of current sheets, and the degree of mixing between the shocked ambient medium and the relativistic pulsar wind material.

3D dynamics and morphology of bow-shock pulsar wind nebulae

Bow-shock pulsar wind nebulae (PWNe) show a variety of morphological shapes. We attribute this diversity to the geometrical factors: relative orientations of the pulsar rotation axis, proper velocity, and the line of sight (magnetic inclination angle may also have a certain influence on the morphology). We identify three basic types of bow-shock nebulae: (i) a "Rifle Bullet" (pulsar spin axis and proper velocity are aligned); (ii) a "Frisbee" (pulsar spin axis and proper velocity are orthogonal with the spin axis lying in the plane of the sky), and (iii) a Cart Wheel" (like frisbee but the spin axis is perpendicular to the plane of the sky). Using 3D RMHD simulations, as well as analytical calculations, we reproduce the key morphological features of the bow-shock PWNe, as well as variations, are seen across different systems. magnetic stresses within the shocked pulsar wind affect the overall structure strongly, producing "whiskers", "tails", "filled-in" and "mushroom" shapes, as well as non-symmetric morphologies. On the other hand, the interstellar medium inhomogeneities and the anisotropy of the energy flux in the pulsar wind have only a mild impact of the PWN morphology. In a few cases, when we clearly identify specific morphological structures, our results do not favor alignment of the pulsar spin axis and proper velocity. Our calculations of the underlying emission processes explain the low synchrotron X-ray efficiency (in terms of the spin-down luminosity) and imply an energetical subdominant contribution of the inverse Compton process.

Modelling the bow shock Pulsar Wind Nebulae propagating through a non-uniform ISM

Many pulsars propagate through the interstellar medium (ISM) with supersonic velocities, and their pulsar winds interact with the interstellar medium (ISM), forming bow shocks and magnetotails (PWN). We model the propagation of pulsars through the inhomogeneous ISM using non-relativistic axisymmetric magneto-hydrodynamic (MHD) simulations. We take into account the wind from the star, and the azimuthal and poloidal components of the magnetic field, and investigate the PWN at different levels of magnetization (the ratio of magnetic to matter energy-densities) in the wind. We consider the interaction of PWN with small-scale and large-scale imhomogeneities in the ISM at different values of magnetization. We conclude that the inhomogeneities in the ISM can change the shapes of the bow shocks and magnetotails at different values of the magnetization.We compare the results of our simulations with the images of the Guitar Nebula and other PWN that show irregularities in the shapes of their bow shocks and magnetotails. We conclude that these irregularities may be caused by the interaction of PWN with the inhomogeneities in the ISM.

Numerical simulations of mass loading in the tails of bow-shock pulsar-wind nebulae

When a pulsar is moving through a partially ionized medium, a fraction of neutral Hydrogen atoms penetrate inside the pulsar wind and can be photo-ionized by the nebula UV radiation. The resulting protons remains attached to the magnetic field of the light leptonic pulsar wind enhancing its inertia and changing the flow dynamics of the wind. We present here the first numerical simulations of such effect in the tails of bow shock nebulae. We produce a set of different models representative of pulsars moving in the interstellar medium with different velocities, from highly subsonic to supersonic, by means of 2D hydrodynamic relativistic simulations. We compare the different tail morphologies with results from theoretical models of mass loading in bow shocks. As predicted by analytical models we observe a fast sideways expansion of the tail with the formation of secondary shocks in the ISM. This effect could be at the origin of the head-and-shoulder morphology observed in many BSPWNe.

Escape of high-energy particles from bow-shock pulsar wind nebulae

The detection of bright X-ray features and large TeV halos around old pulsars that have escaped their parent Supernova Remnants and are interacting directly with the ISM, suggest that high energy particles, more likely high energy pairs, can escape from these systems, and that this escape if far more complex than a simple diffusive model can predict. Here we present for the first time a detailed analysis of how high energy particles escape from the head of the bow shock. In particular we focus our attention on the role of the magnetic field geometry, and the inclination of the pulsar spin axis with respect to the direction of the pulsar kick velocity. We show that asymmetries in the escape pattern of charged particles are common, and they are strongly energy dependent. More interestingly we show that the flow of particles from bow-shock pulsar wind nebulae is likely to be charge separated, which might have profound consequences on the way such flow interacts with the ISM magnetic field, driving local turbulence.

A laminar model for the magnetic field structure in bow-shock pulsar wind nebulae

Bow Shock Pulsar Wind Nebulae are a class of non-thermal sources, that form when the wind of a pulsar moving at supersonic speed interacts with the ambient medium, either the ISM or in a few cases the cold ejecta of the parent supernova. These systems have attracted attention in recent years, because they allow us to investigate the properties of the pulsar wind in a different environment from that of canonical Pulsar Wind Nebulae in Supernova Remnants. However, due to the complexity of the interaction, a full-fledged multidimensional analysis is still laking. We present here a simplified approach, based on Lagrangian tracers, to model the magnetic field structure in these systems, and use it to compute the magnetic field geometry, for various configurations in terms of relative orientation of the magnetic axis, pulsar speed and observer direction. Based on our solutions we have computed a set of radio emission maps, including polarization, to investigate the variety of possible appearances, and how the observed emission pattern can be used to constrain the orientation of the system, and the possible presence of turbulence.

Evolved Pulsar Wind Nebulae as Sources of (Mostly Leptonic) Cosmic Rays

Pulsars, and more precisely their wind nebulae, are perhaps the most likely astrophysical sources of high-energy cosmic-ray electrons and especially positrons, and in particular of the excess positronic component above ∼20 GeV measured by PAMELA, Fermi-LAT and AMS-02. While e± pairs are created in pulsar magnetospheres, their acceleration to relevant cosmic-ray energies likely occurs at the pulsar wind termination shock, and they are subsequently confined in the pulsar wind nebula (PWN) until late stages of its evolution. We discuss the implications of radiative and adiabatic energy losses for the likely contributions of different stages of PWN evolution to the observed cosmic-ray e± spectrum, and highlight the plausibly dominant contribution of late, subsonic expansion and especially bow-shock PWN phases.The most recently opened observational window on PWNe, with implications for accelerated e± in the relevant energy range, is the domain of TeV γ-rays. I review the properties of the population of PWNe revealed in this energy domain, in particular by the H.E.S.S. array of Cherenkov telescopes and its Galactic Plane Survey, and discuss the constraints which these place on the evolution of middle-aged PWNe. Finally, I briefly review the possibility of hadron acceleration in PWNe, and discuss theoretical and observational constraints which limit a plausible major contribution of these objects to the bulk of the (hadronic) Galactic cosmic-ray spectrum.

Bow-shock pulsar-wind nebulae passing through density discontinuities

Bow-shock pulsar wind nebulae are a subset of pulsar wind nebulae that form when the pulsar has high velocity due to the natal kick during the supernova explosion. The interaction between the relativistic wind from the fast-moving pulsar and the interstellar medium produces a bow-shock and a trail, which are detectable in H$_{\alpha}$ emission. Among such bow-shock pulsar wind nebulae, the Guitar Nebula stands out for its peculiar morphology, which consists of a prominent bow-shock head and a series of bubbles further behind. We present a scenario in which multiple bubbles can be produced when the pulsar encounters a series of density discontinuities in the ISM. We tested the scenario using 2-D and 3-D hydrodynamic simulations. The shape of the guitar nebula can be reproduced if the pulsar traversed a region of declining low density. We also show that if a pulsar encounters an inclined density discontinuity, it produces an asymmetric bow-shock head, consistent with observations of the bow-shock of the millisecond pulsar J2124-3358.